1. Field of the Invention
The present invention relates to three-dimensional terrain-information generating systems and methods and computer programs therefor for generating terrain information including the relief of the ground. In particular, the present invention relates to a three-dimensional terrain-information generating system and method and a computer program therefor for extracting the three-dimensional shape of the ground on the basis of altitude information of the ground, which is obtained by an airplane, a satellite, or the like.
More particularly, the present invention relates to a three-dimensional terrain-information generating system and method and a computer program therefor for extracting the three-dimensional shape of the ground using altitude information mapped onto a two-dimensional plane and building/road information. In particular, the present invention relates to a three-dimensional terrain-information generating system and method and a computer program therefor for extracting the three-dimensional shape of the ground from which buildings are eliminated.
2. Description of the Related Art
With the recent innovation of information technology, various types of information content are created and edited on computers to offer services such as information storage and information distribution. For example, map information indicating buildings and roads is integrated with geographic information on a computer to offer services that present regional information using map images for road guidance and tourist information. Also, real-time navigation services have been offered to mobile stations such as vehicles and ships using a user's current position information detected by GPS (Global Positioning System) or the like.
A map, landform, or geographic information is not only obtained by measurement on the ground, but also created on the basis of the results of observation from space by an airplane, a satellite, or the like. A recent airplane with a range sensor can compute the three-dimensional shape of the ground on the basis of measured altitude information of the ground. Maps, in general, are orthographic projections, whereas aerial photographs and satellite photographs are central projections. Altitude information measured from space is compensated for geographical errors, and the altitude information is made into an ortho-image on the basis of the accurate geographic information, allowing the altitude information to be mapped onto observation points on the map. (In the description, “elevation-data” refers to data generated by mapping altitude information to each observation point on a two-dimensional plane.)
Basically, map information is two-dimensional planar position information. The integration of the map information with such altitude information allows the relief of the ground to be represented. As a result, for example, a navigation system displays a stereoscopic three-dimensional map image by taking the relief of the ground and landscape into consideration. Such higher quality map information display services are thus offered. Alternatively, the three-dimensional map information is applied to public services such as flood control simulation or systems using virtual space.
In the suburbs and mountains, measurement of the range from space to the ground directly and easily generates altitude information covering an extensive region. On the other hand, in central cities and downtown areas crowded with buildings, the distance, not to the ground, but to ground covering objects such as houses and buildings is measured, resulting in an error component of terrain information.
In the use of altitude information (z) disposed on a plane (x, y), the altitude information is subjected to a moving average filter or the like to minimize the effects of unevenness due to ground covering objects such as buildings. Alternatively, altitude information of regions corresponding to building regions that are obtained from a building map is eliminated. Subsequently, each piece of the altitude information serves as a spatial point at (x, y, z). The adjacent spatial points are connected to create a polygon mesh, and polygon reduction is performed to obtain the three-dimensional shape of the ground.
The above-described techniques are disadvantageous in that the buildings' altitude information is included in terrain data.